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(11) | EP 0 659 468 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Filter element and method for the production thereof |
| (57) A filter element includes a bundle (1) of hollow fibres made of a first polyolefin.
The bundle (1) has at least one end sealed with a second polyolefin (2) having an
average molecular weight in the range of 1,000 to 16,000. The filter element is produced
by a method including the steps of fusing the second polyolefin (2) having a fusion
point lower than that of the first polyolefin (1), attaching the fused second polyolefin
(2) to at least one end of the bundle, allowing the attached second polyolefin to
set, thereby providing the bundle with a sealed portion, and cutting off the leading
end of the sealed portion. |
[0001] The present invention relates to a filter element, which is highly resistant to solvents and comprises a bundle of hollow fibres having at least one end sealed with a thermoplastic resin material, and relates also to a method for the production thereof.
[0002] In conventional filter elements of this kind, at least one end of a bundle of hollow fibres is sealed with urethane adhesive. Use of such urethane adhesive leads to good workability when providing the at least one end of the bundle with a sealed portion. A filter module having such a conventional filter element contained in a housing exhibits sufficient durability to water. However, it is inferior in durability to an aqueous alcohol solution or organic solvents such as alcohol.
[0003] In view of the above, filter elements each comprising a bundle of hollow fibres which has at least one end sealed with a thermoplastic resin material resistant to solvents have recently been developed.
[0004] Such filter elements are disclosed, for example, in Japanese Patent Non-examined Publications No. 1-218605, No. 3-245826 and No. 4-63117.
[0005] However, the use of a thermoplastic resin has disadvantages in (1) that the step of forming a sealed portion requires a treating temperature higher than in the case of using urethane adhesive in order to obtain fluidity of the resin, resulting in thermal deformation of the hollow fibres and possible destruction of the inside diameters of the hollow fibres depending on a combination with the material of which the hollow fibres are made, (2) that since thermoplastic resin has viscosity higher than that of urethane adhesive, it fails to propagate over the entire spaces between the adjacent hollow fibres even if it should gain fluidity, resulting in an incomplete sealed portion, and (3) that thermoplastic resin encounters volume shrinkage larger than that of urethane adhesive during its setting, resulting in an incomplete sealed portion having crazing and/or cavities.
[0006] The first cited Japanese Publication discloses a method for the formation of a sealed portion of a bundle of hollow fibres by filling the inside of the ends of the hollow fibres with calcium carbonate so as not to induce thermal deformation, then sealing the spaces between the adjacent ends of the hollow fibres with fine particles of thermoplastic resin the same as the thermoplastic resin of which the hollow fibres are made, and fusion bonding the ends of the hollow fibres. This method requires the steps of removing the calcium carbonate by dissolving it in concentrated hydrochloric acid which will constitute waste matter and disposing of the waste matter, and encounters some difficulty in fusion bonding the ends of the hollow fibres liquid-tightly over the entire area using finely particulate sealing material and failure to fusion bond the centre portion of the bundle of hollow fibres. Thus, this prior art cannot eliminate the disadvantages (2) and (3) mentioned above.
[0007] The second cited Japanese Publication discloses a method for the formation of a sealed portion of a bundle of hollow fibres, which adopts the step of causing the sealed portion to set from the lower part thereof to eliminate the aforesaid disadvantage (3) and uses, as a material for hollow fibres, engineering plastic generally exhibiting high resistance to heat, including polysulfone, polyether sulfone and polyamide and, as a sealing material, thermoplastic resin including polyolefin, polyvinyl chloride, nylon and polyester. This method requires that the fusion point of the material for hollow fibres should be much higher than that of the sealing material and cannot eliminate the aforesaid disadvantage (2).
[0008] The third cited Japanese Publication discloses a method for the formation of a sealed portion of a bundle of hollow fibres, which uses a first polyolefin as a material for hollow fibres and, as a sealing material, a second polyolefin having a fusion point lower than that of the first polyolefin, and adopts the use of a suspension of the second polyolefin for sealing the ends of the hollow fibres. The products obtained in accordance with this method, however, are not stable in liquid tightness. Thus, this method produces no satisfactory results.
[0009] The present invention has been proposed for the purpose of solving the problems mentioned above.
[0010] An object of this invention is to provide a filter element highly resistant to solvents and chemicals.
[0011] Another object of this invention is to provide a method for the production of the filter element at low cost.
[0012] According to one aspect of this invention, there is provided a filter element comprising a bundle of hollow fibres made of a first polyolefin, the filter element being characterised in that the bundle has at least one end sealed with a second polyolefin having an average molecular weight in the range of 1,000 to 16,000.
[0013] According to another aspect of this invention, there is provided a method for the production of a filter element comprising a bundle of fibres made of a first polyolefin having a fusion point, the method being characterised by the steps of fusing a second polyolefin having an average molecular weight in the range of 1,000 to 16,000 and a fusion point lower than the fusion point of the first polyolefin, attaching the fused second polyolefin to at least one end of the bundle, allowing the attached second polyolefin to set, thereby providing the bundle with a sealed portion, and cutting off a leading end of the sealed portion.
[0014] Embodiments of this invention will be described hereinbelow with reference to the accompanying drawings, in which:
Figure 1 is an explanatory cross section showing one embodiment of a method for producing a filter element according to this invention;
Figure 2 is a cross section showing the filter element produced by the method; and
Figure 3 is an explanatory cross section showing another embodiment of a method for producing a filter element according to this invention.
[0016] A bundle 1 of hollow fibres made of a first polyolefin is prepared. A second polyolefin having an average molecular weight in the range of 1,000 to 16,000 and a fusion point lower than that of the first polyolefin is fused in a mould 3 by heating the mould with a heating means 4. The difference between the fusion points of the first and second polyolefins falls within 30°C. The fused second polyolefin 2 in the mould 3 is left standing until it has a temperature not higher than the fusion point of the first polyolefin while maintaining its fluidity. The prepared bundle 1 is then immersed in the fused second polyolefin 2 as shown in Figure 1. The bundle 1 is removed from the mould 3 immediately before the fused second polyolefin 2 loses its fluidity, and the second polyolefin attached to the bundle 1 is allowed to set. As a result, a filter element 9 having a sealed portion 10 of the second polyolefin is obtained as shown in Figure 2.
[0017] The second polyolefin includes low-density polyethylene having an average molecular weight of 6,000 or less, high-density polyethylene having an average molecular weight of 7,000 or less, and polypropylene having an average molecular weight of 16,000 or less. Polypropylene or high-density polyethylene is preferable when a filter element being obtained is required to be sterilised with steam. Since the second polyolefin in a fused state exhibits lower viscosity than ordinary polyolefin, it can be fused by heating the mould 3 to a temperature of 200°C and holding the mould 3 at that temperature for several minutes, and the fused second polyolefin contains no air bubbles therein and is transparent and homogenous. Therefore, it is unnecessary to subject the fused second polyolefin to degassing under a reduced pressure. Where any problem may possibly arise when air is contained in the fused second polyolefin, the degassing step may be adopted.
[0018] When the hollow fibres of the bundle 1 are made of high-density polyethylene, high-density polyethylene having a low molecular weight or low-density polyethylene having a low molecular weight is preferably used as the second polyolefin. When the hollow fibres are made of polypropylene, polypropylene having a low molecular weight, high-density polyethylene having a low molecular weight or low-density polyethylene having a low molecular weight is preferably used as the second polyolefin. These combinations of the first and second polyolefins can avoid thermal deformation of the bundle 1 of hollow fibres and produce a completely sealed portion 10 having the second polyolefin propagating entirely over at least one end of the bundle 1.
[0019] As described above, the bundle 1 is removed from the mould 3 immediately before the fused second polyolefin 2 loses its fluidity, and the second polyolefin attached to the bundle 1 is allowed to set outside the mould 3. The removal of the bundle 1 from the mould 3 is carried out preferably when the temperature of the mould 3 is in the range between the fusion point of the second poloyolefin and a temperature not exceeding the temperature 10°C higher than the fusion point of the second polyolefin. By so doing, the second polyolefin can firmly be attached to the bundle 1. In addition, since the second polyolefin attached to the bundle 1 is allowed to set outside the mould 3, there is no possibility of the sealed portion 10 suffering from crazing and/or cavities by virtue of volume shrinkage of the second polyolefin. Therefore, a filter element having at least one end sealed with the second polyolefin can be obtained.
[0020] In order to obtain a filter element having a more completely sealed portion, the filter element 9 shown in Figure 2 is inserted at its sealed portion 10 into another mould 7 having a bottomless nylon cup 6 attached to the inside thereof and a heating means 8 attached to the outside thereof as shown in Figure 3. The mould 7 contains either fused polyolefin of a low molecular weight which may be the same as the second polyolefin or a fused mixture of the low molecular weight polyolefin and ordinary polyolefin having an average molecular weight larger than that of the low molecular weight polyolefin. The filter element 9 is removed from the mould 7 and allowed to set in the same manner as described above. The filter element 9 thus obtained is associated with a given support cylinder via an O-ring or the like into a filter module.
[0021] A given support cylinder made of ordinary polyolefin of the same type as the low molecular weight polyolefin may be inserted into the mould 7. In this case, the filter element being obtained can be used as a filter module.
[0022] In a case where the mould 7 contains a fused low molecular weight polyolefin and where a given support cylinder is not inserted into the mould 7, there is a possibility that a filter element being obtained has a sealed portion liable to break when receiving a large external impact. This can be avoided by using the aforementioned fused mixture instead of the fused low molecular weight polyolefin.
[0023] Examples of the ordinary polyolefin include low-density polyethylene, linear low-density polyethylene, high-density polyethylene and polypropylene. The content of the ordinary polyolefin in the mixture is preferably in the range of 10 to 30% by weight.
[0024] According to the method of this invention, even when hollow fibres constituting a bundle are made of engineering plastic having a higher fusion point than polypropylene, use of the low molecular weight polyolefin can eliminate otherwise possible thermal deformation of the bundle and provide a completely sealed portion having the low molecular weight polyolefin fully propagating the spaces between the adjacent hollow fibres.
[0025] The advantages of the present invention will become more apparent from a working example and comparative experiments which will be described hereinafter.
Example 1
[0026] A bundle 1 was prepared by bundling 3,000 hollow fibres made of polypropylene. A mould 3 containing high-density polyethylene having an average molecular weight of 3,000 was heated by a heater 4 until the fused polyolefin had no air bubbles therein. The application of heat was then stopped and the mould 3 was left standing. As soon as the temperature of the mould 3 was 160°C, the bundle 1 was inserted into the fused polyolefin and left standing. When the mould temperature became 120°C, then the bundle 1 was removed from the mould 3 to obtain a filter element 9 having a sealed portion 10 at one end thereof.
[0027] Another mould 7 having a bottomless nylon cup 6 attached fast to the inside thereof and a mixture 5 of high-density polyethylene having an average molecular weight of 3,000 and 20% by weight of straight-chain polyethylene (MFR80 as specified under ASTM) contained inside the cup 6 was heated by a heater 8 until the fused mixture 5 had no air bubbles therein. The application of heat was then stopped and the mould 7 was left standing. As soon as the mould temperature was 130°C, the sealed portion 10 of the filter element 9 was inserted into the fused mixture 5. When the temperature of the mould 7 became 100°C, then the filter element 9 having the cup 6 attached to the further sealed portion thereof was removed from the mould 7 and allowed to set. After the further sealed portion had set sufficiently, the attached cup 6 was removed. As a result, a filter element 9 was obtained having the end portion 10 sealed completely with polyolefin. Neither crazing nor cavities could be observed in the sealed portion 10.
[0028] The leading end of the sealed portion 10 of the filter element 9 thus obtained was cut off to form an open surface of the filter element 9. The filter element 9 was fixed to the inside of a pressure container with the sealed portion 10 sealed with an O-ring. Then, water pressure of 4 kg/cm² was applied to the filter element 9. However, no leak of water from between the hollow fibres could be found at the cut open surface of the filter element 9.
[0029] With the filter element 9 fixed inside the pressure container, 1O l of ethyl alcohol was filtered. Then, the filter element 9 was removed from the pressure container, dried in an oven heated to 60°C, and fixed again to the inside of the pressure container. Thereafter water pressure of 4 kg/cm² was applied to the filter element 9. However, no leak of water could be found at the cut open surface of the filter element 9. Although this test was repeated five times, no leak of water could be recognised at the cut open surface of the filter element 9.
Comparative Experiment 1
[0030] A bundle 1 was prepared in the same manner as in Example 1 and the same procedure as in Example 1 was taken until the insertion of the bundle 1 into the fused polyolefin. When the temperature of the mould 3 became room temperature, the bundle 1 is removed from the mould 3 to obtain a filter element 9 having a sealed portion 10 at one end thereof.
[0031] The leading end of the sealed portion 10 of the filter element 9 thus obtained was cut off to form an open surface of the filter element 9. As a result, crazing was recognised in the cut open surface.
[0032] The filter element 9 was fixed in the same manner as in Example 1 and subjected to water pressure of 4 kg/cm². As a result, leak of water from between the hollow fibres was found at the cut open surface of the filter element 9.
Comparative Experiment 2
[0033] A bundle 1 was prepared in the same manner as in Example 1 and a filter element 9 having a sealed portion 10 was obtained in the same manner as in Example 1 except that urethane adhesive was used as the sealing material.
[0034] The leading end of the sealed portion 10 of the filter element 9 thus obtained was cut off to form an open surface of the filter element 9. As a result, crazing was recognised in the cut open surface.
[0035] The filter element 9 was fixed in the same manner as in Example 1 and subjected to water pressure of 4 kg/cm². As a result, no leak of water' from between the hollow fibres could be found at the cut open surface of the filter element 9.
[0036] The same test as in Example 1 effecting filtration of ethyl alcohol and application of water pressure of 4 kg/cm² was repeated. As a result, after the second and subsequent tests, leak of water from between the hollow fibres was recognised at the cut open surface of the filter element 9.
[0037] As is clear from Example 1 and Comparative Experiments 1 and 2, according to the present invention, it is possible to provide a filter element made of polypropylene applicable to filtration of not only water but also an organic solvent, more inexpensive than filter elements made of an engineering plastic material, and excellent in quality and reproducibility without forming crazing and/or cavities in its sealed portion.
1. A filter element (9) comprising a bundle (1) of hollow fibres made of a first polyolefin,
said filter element being characterised in that said bundle has at least one end sealed
with a second polyolefin (2) having an average molecular weight in the range of 1,000
to 16,000.
2. A filter element according to claim 1, characterised in that said at least one end
of said bundle is sealed with a mixture of ordinary polyolefin and said second polyolefin.
3. A filter element according to claim 1 or 2, characterised in that said first polyolefin
is polypropylene or high-density polyethylene.
4. A method for the production of a filter element (9) comprising a bundle (1) of fibres
made of a first polyolefin having a fusion point, said method being characterised
by the steps of (a) fusing a second polyolefin having an average molecular weight
in the range of 1,000 to 16,000 and a fusion point lower than the fusion point of
said first polyolefin, (b) attaching the fused second polyolefin (2) to at least one
end of said bundle, (c) allowing the attached second polyolefin to set, thereby providing
said bundle with a sealed portion (10), and (d) cutting off a leading end of said
sealed portion.
5. A method according to claim 4, characterised in that the step (a) is carried out in
a mould (3), that the step (b) is carried out by immersing said at least one end of
said bundle (1) into the fused second polyolefin (2) in said mould, and that the step
(c) is carried out after said bundle is removed from said mould immediately before
the fused second polyolefin sets, with the further steps of (e) immersing said sealed
portion (10) of said bundle into a separately prepared fused second polyolefin (5)
contained in another mould (7), and (f) allowing said sealed portion to set, being
carried out before the step (d).
6. A method according to claim 5, characterised in that said another mould (7) further
contains fused ordinary polyolefin admixed with said separately prepared fused second
polyolefin (5).
7. A method according to any one of claims 4 to 6, characterised in that said first polyolefin
is polypropylene or high-density polyethylene.
8. A method according to any one of claims 4 to 7, characterised in that a difference
between the fusion point of said first polyolefin and that of said second polyolefin
falls within 30°C.